Use of a linear solenoid as a drive device of various apparatuses is known. For example, JP2005-45217A (corresponding to US 2004/0257185A1) teaches the use of the linear solenoid as a drive device of a valve timing control apparatus placed in an engine room of a vehicle. In this instance, a profile of the linear solenoid is reduced by reducing an axial size of the linear solenoid to enable installation of the linear solenoid in a limited space.
By reducing the profile of the linear solenoid, an axial length of a plunger is reduced. The plunger is placed on a radially inner side of a front stator and a rear stator. When a coil, which is placed on a radially outer side of the front stator and the rear stator, is energized, a magnetic force is generated. Thus, a magnetic flux flows through the rear stator, the plunger and the front stator, and thereby a magnetic attractive force is generated in the plunger. The plunger is axially displaced from the rear stator side toward the front stator side in the stroke thereof by the magnetic attractive force generated in the plunger. Here, an axial overlapped surface area (a magnetic flux transferring surface area) between the plunger and the rear stator is reduced when the amount of stroke (i.e., the amount of displacement) of the plunger is increased. Therefore, in a latter half of the stroke of the plunger, in which the amount of stroke of the plunger becomes large, the magnetic flux transferring surface area between the plunger and the rear stator becomes small, and thereby a density of the magnetic flux, which flows through the plunger and the rear stator, becomes high.
A radially outward attractive force and a negative attractive force (an attractive force, which has a vector in a direction opposite from an attracting direction of the plunger attracted toward the front stator) are generated in an end portion of the plunger, which is axially located on a side opposite from the front stator. Particularly, in the latter half of the stroke of the plunger, the density of the magnetic flux, which flow through the plunger, becomes high, so that the negative attractive force becomes large. The negative attractive force acts as a force, which pulls back the plunger in the direction opposite from the attracting direction of the plunger toward the front stator. Therefore, the negative attractive force reduces a total attractive force of the linear solenoid to cause a reduction of a magnetic efficiency. Thus, particularly, in the latter half of the stroke of the plunger, the drive force of the linear solenoid, which is applied to a drive subject to drive the same, may possibly be reduced.
In contrast, in order to improve the installability of the linear solenoid in the limited space, such as the engine room, it is desirable to further reduce the profile of the linear solenoid. When the profile of the linear solenoid is further reduced, the axial length of the plunger becomes shorter. Therefore, in the latter half of the stroke of the plunger, the density of the magnetic flux is further increased to cause an increase in the negative attractive force. As a result, the total attractive force of the linear solenoid may possibly be further reduced.